The North claims nearby waters and considers such drills an infringement of its territory. The assault last month was the first by Pyongyang to target a civilian area since the end of the 1950-53 war.

The shelling provoked anger and shock in the South, where TV screens and newspapers were filled with images of islanders fleeing their bombed-out, burning homes.

A senior North Korean military official said in comments published by the North's official Korean Central News Agency that if the South carried out further drills on Yeonpyeong "unpredictable self-defensive strikes will be made".

"The intensity and scope of the strike will be more serious than the [earlier shelling]," the official said in the notice sent to Seoul today.

Pyongyang said the planned drills were an attempt "to save the face of the South Korean military, which met a disgraceful fiasco" during last month's clash.

The South Korean president, Lee Myung-bak, has faced criticism that troops were unprepared for the strike on Yeonpyeong and reacted too slowly and weakly. He has since replaced his defence minister, Kim Tae-young, and pledged to increase the number of soldiers and weapons on islands along the Koreas' disputed western sea border.

Representatives of the US-led UN command, which oversaw the armistice that ended the Korean war, will observe the drills.

The war of words between the Koreas came as the governor of New Mexico arrived in Pyongyang today on an unofficial diplomatic mission to try to ease regional tensions.

Bill Richardson, an unofficial envoy to the North, said he wanted to visit the main nuclear complex and meet senior officials during his four-day trip, though details of his schedule were unclear. He said before the visit: "My objective is to see if we can reduce the tension in the Korean peninsula."

In Washington, the state department spokesman, PJ Crowley, said the South's artillery drills posed no threat to the North. "North Korea should not see these South Korean actions as a provocation," he said.

However General James Cartwright, vice chairman of the joint chiefs of staff, warned the exercises could trigger a chain reaction of firing and counter-firing. "What you don't want to have happen out of that is for us to lose control of the escalation. That's the concern."

Amid the rising tensions, US diplomats held meetings in the region. In Beijing, the US deputy secretary of state, James Steinberg, held private meetings with the Chinese state councillor, Dai Bingguo.

China's head of foreign policy returned last week from talks in Pyongyang with the North Korean leader, Kim Jong-il. China has come under renewed pressure to persuade North Korea, an ally, to change its behaviour.

Pyongyang is believed to be seeking exclusive talks with the US before returning to six-nation nuclear disarmament negotiations hosted by China, which will include South Korea, Japan, and Russia.

Crowley has warned North Korea must cease provocations, reduce tensions in the region, improve ties with South Korea and abandon its nuclear programme before any discussions can take place.